As countermeasures to the rapid increase of crude oil price in recent years and to the depletion of oil resources expected to appear in the near future, development of deep oil fields which did not draw attention and development of highly corrosive sour gas fields and the like which were once abandoned in their development are emphasized over the world. Those kinds of oil fields and gas fields are generally very deep, and have environments of high temperature and highly corrosive, containing CO2, Cl−, and the like. Accordingly, linepipes used for transporting crude oil and gas produced from those kinds of oil fields and gas fields are requested to use steel pipes having high strength and high toughness, and further having excellent corrosion resistance. In addition, development of offshore oil fields has been vigorously progressed, thus the steel pipes in these oil fields are requested also to have excellent weldability in view of reduction in the pipeline laying cost.
Conventional linepipes adopted carbon steels from the point to assure weldability under environments containing CO2 and Cl−1, while separately applying an inhibitor for preventing corrosion. Since, however, inhibitors raise problems of insufficient effect at elevated temperatures and of inducing pollution, their use has been reduced in recent years. Some of the pipelines adopt duplex stainless steel pipes. Although the duplex stainless steel pipes have excellent corrosion resistance, they contain large amounts of alloying elements, are inferior in hot-workability to accept only special hot-working methods for their manufacture, and are expensive. Consequently, the use of stainless steel pipes is rather limited at present. With these problems, industries wait for steel pipes for linepipes having excellent weldability and corrosion resistance, at low price.
Responding to the requirement, there are proposed 11% Cr or 12% Cr martensitic stainless steel pipes that improve the weldability for linepipe services, disclosed in, for example, Unexamined Japanese Patent Publication No. 08-41599, Unexamined Japanese Patent Publication No. 09-228001, and Unexamined Japanese Patent Publication No. 09-316611.
The steel pipe disclosed in JP '599 is a martensitic stainless steel pipe for linepipes, having excellent corrosion resistance at welded part by decreasing carbon content to control the increase in the hardness of the welded part. The steel pipe disclosed in JP '001 is a martensitic stainless steel pipe, which increases the corrosion resistance by adjusting the amounts of alloying elements. The steel pipe disclosed in JP '611 is a martensitic stainless steel pipe for linepipes, which satisfies both the weldability and the corrosion resistance.